The present invention relates generally to wireless communication systems and, more particularly, to a method for synchronizing mobile terminals within such communication systems.
The number of mobile terminals in use has grown rapidly over the past decade. However, the available bandwidth to accommodate the increasing number of users is essentially fixed. Consequently, much effort is devoted to devising ways to improve the spectral efficiency of wireless communication systems so that the demand for wireless services can be met. For example, a variety of multiple access schemes have been devised that allow multiple users to share the same radio channel carrier frequency. One common multiple access scheme is known as Time Division Multiple Access, or TDMA. A wireless communication system that employs TDMA is referred to herein as a TDMA system.
In a TDMA system, each carrier frequency is divided into repeating frames. The frames are subdivided into a plurality of time slots. A mobile terminal is assigned one or more slots on separate transmit and receive frequencies. Assigned slots on both transmit and receive frequencies represent a communications channel. Wireless terminals transmit signals to the serving base station in short bursts in their assigned slot(s) on the transmit frequency and receive transmitted bursts from the serving base station in their assigned slot(s) on the receive frequency. Thus, each pair of frequencies (transmit and receive) can support a number of users equal to the number of slots in a frame. In this manner, TDMA-based systems greatly increase the number of individual users supported by the defined set of frequencies.
Mobile terminals operating in a TDMA system must synchronize with the base station and avoid interfering with other mobile terminals operating on the same frequency. Information transmitted by the base station in each time slot includes a predefined synchronization word (sync word). By identifying the location of a sync word in a received signal, a mobile terminal can synchronize with the base station. Ideally, the mobile terminal performs initial synchronization after powering up by selecting a frequency on which the base station is transmitting, receiving a portion of a signal transmitted by the base station for a period of time greater than at least one TDMA slot, and identifying the location of the predefined sync word within the received signal. To do this, the mobile terminal typically mixes the received signal down to a baseband frequency and samples this baseband signal at a rate corresponding to the symbol rate of the transmitted signal to generate a signal sample set. The signal sample set is then correlated with a known sync word pattern to determine the location of the sync word in the signal sample set. However, successfully determining the location of the sync word depends on achieving an acceptable correlation threshold between one of a defined set of sync words or patterns and a subset of the signal sample set.
Sampling the baseband signal to generate the signal sample set used for sync word correlation should be done at the ideal symbol period sampling point where the sample instances are at the peak response of the symbol. However, the sampling phase corresponding to this ideal symbol sampling point is unknown to the mobile terminal at initial synchronization. Additionally, any offset in the mobile terminal""s local oscillator (LO) frequency is unknown. This LO frequency is used to mix the received signal down to baseband frequency and LO frequency offsets cause corresponding offsets in actual baseband signal frequency. Often, the magnitude of possible LO frequency offset error greatly exceeds the offset error permissible in the baseband frequency for successful sync word correlation. The frequency span of possible baseband offset error must then be divided into a set of trial frequencies separated by frequency steps that do not exceed the magnitude of permissible offset error. Since the actual LO offset is unknown, the signal sample set must be compensated using the set of trial frequencies. Sync word correlation must be performed using the signal sample set compensated for each trial frequency until an acceptable correlation threshold is found, resulting in a computationally expensive and time consuming set of operations. In some implementations, multiple signal sample sets are generated at different sampling phases. In this case, the above set of trial frequency compensations and correlation calculations must be performed for each one of the multiple signal sample sets.
As an example of initial synchronization processing, a particular Digital Advanced Mobile Phone System (D-AMPS) mobile terminal captures a block of 1600 complex samples of the received signal after it is converted to the baseband frequency. The samples are taken at eight times the transmitted symbol rate. This set of 1600 samples is arbitrarily downsampled to a single sample per received symbol, which leaves one set of 200 symbol-rate samples (1600/8=200). For successful sync word correlation, the mobile terminal must compensate the selected symbol-rate sample set for baseband offset frequency. The mobile terminal must apply compensation to within xc2x1300 Hz of the actual baseband offset frequency. Since the offset frequency is unknown and can be as much as xc2x19 KHz, the mobile terminal creates a set of trial frequencies, spanning the 18 KHz range in 600 Hz steps. The mobile terminal then performs correlation operations using the selected symbol-rate sample set compensated by each one of these trial frequencies until an acceptable sync word correlation threshold is achieved. The mobile terminal performs up to 31 such correlation operations, one for each of the trial frequencies. Because D-AMPS uses multiple sync words, the above process may not result in an acceptable correlation threshold being achieved for the sync word initially chosen by the mobile terminal. If not, the mobile terminal repeats the trial frequency compensation and correlation operations for each possible sync word until an acceptable correlation threshold is achieved. If the mobile terminal successfully correlates one of the predefined sync words with the captured data set, it captures a new set of baseband data using predefined slot timing to verify location of the same sync word a second time. This second verification completes the initial synchronization process.
The computational complexity of the initial synchronization can result in significant delays in acquiring service when the mobile phone is initially turned on. Therefore, a less computationally intensive synchronization method would be desirable.
The present invention provides a method and apparatus for reducing the complexity of initial synchronization in a mobile terminal operating in a TDMA system. A mobile terminal receives a downlink transmission from a base station for a period greater than one TDMA time-slot. Symbol-encoded information transmitted by the base station during this period includes at least one predefined sync word. Using a local oscillator having an unknown frequency offset error, the mobile terminal mixes the received signal down to a baseband frequency. The frequency offset error is therefore impressed onto the baseband signal. Sampling this baseband signal at a rate M times faster than the transmitted symbol rate, the mobile terminal generates M symbol-rate sample sets having M different sampling phases. The mobile terminal performs a Fast Fourier Transform (FFT) on each one of the M symbol-rate sample sets after raising it to the nth power to remove the effect of signal modulation. The value of n depends on the number of modulation phases used in transmission. For example, with Quadrature Phase-Shift Keying (QPSK) modulation, n equals 4. The mobile terminal compares the peak magnitude from each FFT and selects the sample set corresponding to the greatest peak value. The selected sample set represents the symbol-rate sample set having the preferred sampling phase. The frequency at which the peak FFT magnitude occurs corresponds to the LO offset frequency times n, which is a result of passing the sample set through the nth law device. However, in D-AMPS, n times the actual LO offset frequency may result in a frequency above the Nyquist limit imposed by the baseband FFT sampling rate. Therefore, the offset frequency found in the FFT data set having the greatest peak magnitude may be n times the actual LO offset frequency, or may be one of a small number of possible aliases. In D-AMPS, as in any practical system, the possible range of LO offset is limited by design. Therefore, alias frequencies outside this possible range are discarded and the remaining small set of frequencies is used to compensate the selected baseband symbol-rate sample set in sync word correlation operations. In the earlier D-AMPS example, the set of trial frequencies includes 31 different compensation frequencies. The present invention reduces the set of trial frequencies to three, thereby reducing the complexity of the calculations.